Wire bonding is a method used in the semiconductor industry to attach a fine wire, commonly 1 to 3 mils in diameter, from one connection pad to another to complete an electrical connection between electronic devices. The most widely used wire materials are Gold (Au) and Aluminum (Al), but Silver (Ag) and Copper (Cu) are also used. The connection pads may comprise metallized bond sites on a semiconductor chip or on interconnection substrates. A semiconductor chip can also be wire bonded to a metal leadframe as is done in plastic encapsulated devices.
A typical method used to bond or weld the wire to a connection pad is through a combination of heat, pressure and/or ultrasonic energy. It is a solid phase welding process, wherein the two metallic materials (the wire and the pad surface) are brought into intimate contact. Once the surfaces are in intimate contact, electron sharing or interdiffusion of atoms takes place, resulting in the formation of a wire bond. The bonding force can lead to material deformation, breaking up of a contamination layer and smoothing out of surface asperity, which can be enhanced by the application of ultrasonic energy. Heat can accelerate inter-atomic diffusion, thus forming the bond.
One type of wire bond formation uses a ball bond. The process involves melting a sphere of wire material on a length of wire held by a capillary, which is lowered and welded to a first bonding position. The capillary then draws out a loop and then connects the wire to a second bond position using a wedge bond that is usually of a crescent shape. Another ball is then reformed for a subsequent first ball bond. Currently, gold ball bonding is the most widely used bonding technique. Its advantage is that once the ball bond is made on the connection pad of a device, the wire may be moved in any direction without stress on the wire, which greatly facilitates automatic wire bonding.
Current wire bonding techniques depend very much on the area of contact between the formed ball and the connection pad of the electronic device for adequately securing the connection. Over the years, the demand for fine-pitch bonding (such as with wires having diameters of less than 50 μm) has increased steadily, thus making effective bonding more difficult since there is a smaller surface area for contact between the wire bond and the connection pad. Furthermore, probe testing of semiconductor devices has become the norm. Probe testing may cause the surfaces of the connection pads to be damaged, leaving probe marks on the connection pads which might be rough or have an under-layer material exposed, thus adding to the difficulty to form an effective bond since good intermetallization is harder to achieve.
Another problem associated with fine-pitch bonding is that if an insufficient amount of ultrasonic energy or bond force is applied during bonding, ball lift occurs when the adhering force between the ball bond and the connection pad is not strong enough. Conversely, if too much ultrasonic energy or bond force is applied, this may lead to metal peel or cratering on the surface of the connection pad. Moreover, in fine-pitch ball bonding, a parameter window for forming a good bond is comparatively smaller. Therefore, the aforementioned faults would have a tendency to occur either due to the sensitivity of the connection pad of the wafer or other semiconductor device, or due to the parameters not being properly optimized.
In order to improve the intermetallization between the ball bond and the connection pad, one method is to increase the ball size. Unfortunately, the size of the ball is restricted to the size of the opening offered by the connection pad which is smaller for smaller devices. Another method is to increase the ultrasonic energy transmitted to the ball bond during bonding. However, this method increases the risk of metal peel or cratering if the wafer or semiconductor device is sensitive,
As mentioned above, probe marking on the die surface also reduces intermetallization. Thus, rectangular pad opening designs have been adopted over recent years so that bonding on the probe mark can be partially avoided. Even so, there is no certainty that the probe mark can be sufficiently avoided during bonding in order to increase the contact area between the ball bond and the connection pad to achieve better intermetallization. Therefore, prior art bonding methods face obstacles in improving the quality of bond adhesion due to the aforesaid limitations,